1. Field of the Invention
The present invention concerns a mammography method and a mammography apparatus suitable to implement the method.
2. Description of the Prior Art
Mammography is an x-ray examination of the breast with the goal of detecting tumors at an earliest possible stage. Through continuous improvement of mammography techniques it is sought to generate x-ray images with high significance in order to differentiate benign tissue variations from malignant variations and to reduce the number of incorrect findings, i.e. the number of suspicious findings that are caused by non-malignant variations and the number of undetected malignant tumors. In conventional x-ray mammography, a single two-dimensional image of the compressed breast is generated at a single projection direction. Since the tissue slices situated one after another in the direction of the x-ray beam are superimposed in such a projection, strongly absorbent benign structures can overlay a malignant tumor and hinder the ability to detect such a tumor.
In order to avoid this problem, a mammography technique known as tomosynthesis—in which individual images (exposures) of the breast are acquired with a digital x-ray detector in a number of different projection directions—is known from, for example, T. Wu et al., “Tomographic mammography using a limited number of low-dose cone-beam projection images”, Med. Phys. 30, 365 (2003). A number of slice images that each represent a slice of the breast orientated parallel to the acquisition surface of the x-ray detector can then be reconstructed from these digital exposures acquired from different projection angles, i.e. from the image data belonging to these exposures. Such an image data set obtained by reconstruction is designated in the following as a tomosynthetic 3D x-ray image. Tissue structures that are situated deeper (as viewed in the propagation direction of the x-ray beam) can be detected better by using this measure.
Moreover, to improve tumor detection a digital mammography method is known in which a contrast agent that propagates in the blood vessels of the breast is intravenously injected into the patient after the generation of a first two-dimensional image. Such a method is described in Roberta A. Jong et al., “Contrast-enhanced Digital Mammography Initial Clinical Experience”, Radiology 2003, Vol. 228, P. 842-850. The temporal propagation of the contrast agent that increasingly accumulates (enriches) at malignant lesions can be visualized in a number of exposures acquired in the same projection direction after the injection. The kinetics—i.e. the time curve of the enrichment—additionally indicates the presence of a malignant tumor.
A further development of this contrast agent-enhanced digital mammography method in which a first exposure is acquired with a low-energy x-ray beam and a second exposure is acquired with a high-energy x-ray beam, is known from J. M. Lewin et al., “Dual-energy contrast-enhanced digital subtraction mammography: feasibility”, Radiology 2003, Vol. 229, P. 261-268. The energy spectrum of the low-energy x-ray beam is selected so that the contrast agent is practically invisible in that exposure, while the higher-energy x-ray beam is strongly absorbed by the contrast agent. A reference image, from which the structures of the normal breast tissue are largely eliminated and in which the contrast agent (which possibly accumulates only weakly due to the strong compression of the breast) is more clearly visible is generated from these exposures.
The contrast agents that are used are normally toxic and can be linked to unwanted side effects, such that their use should be limited to a minimum.